The present invention relates to a dot matrix printer, and more particularly to an impact type printing head using piezoelectric elements.
A printer of this kind has a plurality of selectively driven printing needles or printing wires to print figures and letters in the form of dot matrixes on paper. Presently, most of the printers of this kind employ electromagnets for driving the printing needles or wires. For example, a line printer disclosed in the U.S. Pat. No. 3,941,051 comprises plate springs each having a printing needle, permanent magnets displaced at the lower rear of the plate springs, and electromagnets at the upper rear of the plate springs. The plate springs are bent by the attracting magnetic force of the permanent magnet. When one of the electromagnets is energized, the magnetic force of the permanent magnet is offset, and the printing needle is projected toward a platen by the resilient force of the plate spring. However, such a conventional printer requires an excessively large driving current due to the heat generated in a coil of the electromagnets, eddy currents and a hysteresis loss occurring in a yoke and an armature. Therefore, the current capacity of a driving circuit for the electromagnets and the capacity of a power source are necessarily large. This causes the printer to be large and expensive. Furthermore, the use of electromagnets and permanent magnets attached to the reciprocating printing mechanism produces a heavy printing mechanism. Therefore, high printing speed cannot easily be realized.
A matrix printer with piezoelectrically driven printing needles has been proposed in the U.S. Pat. No. 4,193,703 for the purpose of decreasing the printer driving current and for increasing the printing speed. A printing mechanism in this printer has two piezoelectric elements fixed to a frame, a buckling spring fastened to one end of each of the piezoelectric elements, and a printing needle provided on the buckling spring. The buckling spring is bent when the piezoelectric elements are expanded in response to an applied voltage. When this occurs, the printing needle projects an extent corresponding to the degree of expansion of the piezoelectric elements. In this printing mechanism, since a very small expansion of the piezoelectric elements is converted directly into the bending of the buckling spring, the printing needle stroke is only about 0.4 mm. This stroke is not long enough to accomplish a quality dot printing operation. Accordingly, the clarity of the printed letters is poor.